Drop on demand inkjet technology for producing printed media has been employed in products such as printers, multifunction products, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of drop generators or inkjets, which are arranged in a printhead or a printhead assembly, onto an image receiving substrate. For example, the printhead assembly and the image receiving substrate may be moved relative to one other and the inkjets may be controlled to emit ink drops at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that activate the inkjets to eject ink. The image receiving substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image receiving substrate may also be a moving continuous web of print medium or sheets of a print medium onto which the ink drops are directly ejected. The ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink, or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid or a gel form and delivered to a melting device, which heats the ink to generate liquid ink that is supplied to a printhead.
Typically, ink drops deposited on an obverse side of a print medium should be affixed to the obverse side without bleeding through or penetrating to a reverse side of the print medium. Penetration of an ink drop through the thickness of a print medium is referred to as “show-through”, because the ink originally deposited on the obverse side is visible on the reverse side of the print medium. Show-through may also occur when an ink drop only partially penetrates the thickness of a print medium, but is still visible on the reverse side. Show-through reduces the image quality of duplex printing operations, which form an image on both sides of the print medium. Specifically, ink deposited on an obverse side of a print medium may at least partially penetrate the print medium and blur or distort an image formed with ink deposited on a reverse side of the print medium. Show-through also affects simplex printing operations, which form an image on only the obverse side of the print medium, because the obverse image density is reduced, and the reverse side of a print medium should generally remain free from ink deposits.
Show-through is related to the properties of the print medium and the ink ejected onto the print medium. For instance, some print mediums have a porous structure that permits an ink to penetrate the print medium before the ink stabilizes, cures, or hardens. Additionally, the viscosity of the ink ejected onto the print medium may result in show-through. For example, inks having a low viscosity are more likely to be absorbed by the print medium than inks having a high viscosity. For these reasons and others, efforts to reduce show-through have been directed to either the properties of the ink or the print medium onto which the ink is deposited.
Processes for reducing show-through are effective, but often limit the type of print medium or ink that may be used by an inkjet printer. For instance, show-through may be reduced or eliminated by coating a print medium with a polymer that makes the print medium have a nonporous surface. The nonporous surface prevents ink deposited onto the print medium from penetrating to a reverse side of the print medium. Polymer coated papers, however, are expensive and certain inks cannot effectively adhere to them. Additionally or alternatively, a printer may be configured to eject only inks having a high viscosity. Many inks, however, do not have a viscosity great enough to prevent the effects of show-through completely, especially on porous print mediums. Accordingly, further developments to reduce show-through are desirable.